Recently, a small group of cysteine tRNA synthetases (CysRSs) was discovered that resembles neither of the two known classes at the primary structural level. Three members from this unique group were revealed from the Methanococcus jannaschii, Deinococcus radiodurans, and Thermotoga maritima. If they function as monomers, then these enzymes represent the smallest known aminoacyl tRNA synthetases (aaRSs) and may be a form of an ancient aaRS. Both in vitro and in vivo CysRS activity was demonstrated. The lack of similarity to all other known synthetases, as well as their modest size, raises questions as to how these enzymes successfully execute their functions and whether they may be an ancient form of what are considered to be among the oldest proteins to have emerged in evolution. My research is designed to study these new enzymes in detail to address the questions as to whether they represent an ancient class of tRNA synthetases and to determine more about how they function My goals are to characterize these enzymes by determining their subunit and domain structure, mapping the tRNA-CysRS complex, and identifying the location and determining the nature of the catalytic site.